The Rose-Red city's age is seven billion years. Let C be the city's
present age; T, the present age of Time. A billion years ago the city
would have been C - 1 billion years old and a billion years from now
Time's age will be T + 1. The data in the puzzle allows us to create two
simple equations:

2C = T

C - 1 = 2/5 (T + 1)

These equations give C, the city's present age, a value of seven billion
years; and T, Time's present age, a value of fourteen billion years.